1. Field of the Invention
This invention relates to an edge detector in an optical measuring instrument for measuring dimensions, displacement values and the like of an opaque body, and more particularly to an edge detector in an optical measuring instrument, wherein an opaque object being measured is directly illuminated by scanning ray beams, a reflected light or a transmitted light due to this illumination, or an image of projection of the object being measured due to the reflected light or the transmitted light are received by photoelectric elements, where electric signals are taken out, and the measurement of dimensions, the judgment of position, the discrimination of contour and the like of the object being measured are carried out in response to the aforesaid electric signals.
2. Description of the Prior Art
Heretofore, the optical measuring instrument of this type such for example as the projector has been of such an arrangement that the object being measured is rested on a mount and illuminated by parallel ray beams, an image of projection of the object being measured is made to focus on a screen by a transmitted light or a reflected light of the parallel ray beams, and the dimensions, contour and the like of the object being measured. However, in general, so-called blurs are found at the edges of the image of the object being measured, which has been projected on the screen. In consequence, it is difficult to accurately read a measured value from the coincidence between the image formed on the screen and a hair line.
To obviate the above-described disadvantage, heretofore, such a method has been adopted that the edge of the formed image is moved relative to a photoelectric element, whereby a variation in value of an electric signal emitted from the photoelectric element is obtained from a variation in ratio of area between a bright portion and a dark portion of the image projected on a light receiving plane of the photoelectric element, so that the variation in the value of the electric signal is compared with a reference voltage, so that the edges of the image of projection can be detected.
However, the above-described method presents the disadvantage that interferences such as external irregular lights affect the performance of the projector to a great extent and the measuring accuracy is greatly deteriorated due to variations in the signal obtained from the photoelectric element and in the reference voltage.
Further, there has been adopted another method wherein a photoelectric element is moved relative to the border or edge of the image projected on the screen, an output signal thus obtained is subjected to second derivative to obtain a wave-form signal, whereby the wave-form signal is compared with a reference voltage to detect the edge. However, this method presents the disadvantages that the position of the edge thus detected may differ depending on the value of the relative movement rates between the photoelectric element and the image of projection, and further, the measuring accuracy is greatly affected by a variation in the reference voltage similarly to the former method.
There has been adopted a still further method, wherein there are provided two photoelectric elements, which are moved relative to the edge of the image of projection, whereby wave-form signal are obtained from a plurality of output signals thus obtained, so that the wave-form signals are compared with the reference voltage to detect the edge. However, this method presents the disadvantages that the measurement becomes very instable due to a relative variation between the output signals obtained from the photoelectric elements and the reference voltage and a variation of the level similar to the above-described methods, further, the scope of application of the illumination ray beams to light intensity is small, and a sensor portion or a circuit portion becomes complicated in construction.
Particularly, in the projector, the brightness of the image projected on the screen is varied due to the fatigue of a light source lamp for illumination, the characteristics of lenses in a projection system and the external irregular lights, and furthere, the brightness suitable for working is varied depending on the color of the pupil of the eye of the measurer (which is different from one race to another) for example, as one of the conditions on the part of measureres, and hence, the brightness must be suitably selected. However, the small scope of application of the illumination ray beams to light intensity results in lowered capacity of the projector.
Furthermore, according to the conventional edge detecting methods, when the focus of the image of projection is shifted, the wave forms emitted from the photoelectric element or elements become gently sloped, thereby presenting the disadvantage that no accurate edge can be detected.
This disadvantage is common in the edge detection among the optical measuring instruments wherein, in general, a transmitted light or a reflected light is detected to directly or indirectly measure dimensions and the like of an object being measured.